Relating to the production of small openings in sheet material

ABSTRACT

In order to form a tapered hole in sheet material, an oversize hole is first formed typically by drilling. The sheet material is then compressed or squeezed in the vicinity of the hole using a press tool to cause the material to flow in the direction of the hole to reduce the size of the hole. The compression produces a hole that tapers from both surfaces of the sheet material to the midpoint of the hole, and an annular region of increased thickness around the area subjected to compression. Machining may be used to remove any annular region of increased thickness so that the remaining sheet material has the same thickness. Further material may be removed by machining to reduce the thickness of the sheet material in the vicinity of the hole so the remaining opening tapers from one surface of the sheet to the other.

REFERENCE TO RELATED APPLICATION

Applicants claim the benefit of the following United Kingdom Patent Applications: Ser. No. 0329400.6, filed Dec. 19, 2003, and Ser. No. 0407370.6 filed Mar. 31, 2004.

1. Field of the Invention

This invention concerns a method and apparatus by which very small openings can be formed in sheet material such as metal foil.

BACKGROUND OF THE INVENTION

Ink-jet printing heads require the formation of very small holes, usually arrays of such holes, in metal foil.

Mechanical drilling is an acceptable method for producing holes having a diameter of 50 μm or greater, but increasing difficulty is experienced below this size. Other processes are available to produce smaller diameter holes, but these present difficulties related to process time, tooling costs etc.

There is a specific requirement for a cost-effective process to manufacture holes in 50 μm thick stainless steel sheet, down to 18μm in diameter.

It is also desirable that such holes taper in one direction, typically at an angle of 15°.

Currently available techniques are described in the paper entitled A Technical Comparison of Micro-electrodischarge machining, Microdrilling and Copper Vapour Laser machining for the Fabrication of Ink Jet Nozzles, David Allen, Heather Almond and Peter Logan, [proceedings of SPIE, Vol. 4019 (2000)]

It is an object of the present invention to provide an improved method and apparatus for making small holes, particularly (but not exclusively) in foil for use in ink jet printing head nozzles.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention an oversize hole is first formed in the sheet material, and after the hole has been produced the sheet material, at least in the immediate vicinity of the hole, is compressed so as to cause the sheet material to flow at least in the direction of the hole, to reduce the size thereof.

Typically the initial hole is formed by drilling.

The initial oversize hole may be for example up to four times larger than the diameter ultimately required of the hole, but the invention is not limited to this ratio and it is to be understood that the initial hole can be any size—the only criteria being that it must be greater in area than that of the final hole.

The compression may be achieved using a press tool.

Typically the tool has a flat underside and is several times larger in diameter than the pre-formed holes, typically five times larger in diameter.

Preferably the tool is positioned concentrically over the pre-formed hole before it is pressed into the material.

The tool material must be chosen so that the tool is not deformed when pressed into the sheet material.

Hardened tool steel and tungsten carbide are examples of a suitable press tool material if the sheet material is stainless steel.

When using a press tool the sheet material will normally be compressed between it and a base plate and just as the tool should not deform under pressure, so the base plate should not yield under the force applied by the tool.

Similar materials as proposed for the tool may be employed for the base plate.

The material displacement which occurs during the compression and deformation of the sheet material will generally cause the material to flow radially, both outwardly of and inwardly towards, the initial hole. The result will be local thinning of the sheet material around the hole, and a reduction in the diameter of the initial hole.

If unrestricted, material displaced radially outwardly by the compression can produce an annular region of increased thickness around the area subject to compression by the press tool.

According therefore to another aspect of the invention, after the pressing step the sheet material may be reduced in thickness by machining so as to remove any annular regions of increased thickness, so that the whole of the sheet material has the same thickness.

So as not to remove more material than necessary, the machining may be arranged simply to remove material from the region of locally increased thickness around the holes caused by material which has flowed radially outwardly from the area subjected to compression, as by the press tool. However this may still leave a depression around the reduced diameter hole corresponding to the shape and size of the lower end of the press tool.

According to another aspect of the invention the machining may be performed so as to reduce the thickness of the whole of the sheet material workpiece to the same thickness as that in the depression surrounding the reduced diameter hole.

Where a press tool is employed, friction between the tool-workpiece (sheet metal) interface can result in the reduced diameter hole assuming a concave barrel shape i.e. its diameter increases from the midpoint of the material thickness towards each surface of the sheet material.

According therefore to a further aspect of the invention further material may be removed by machining so as to reduce the thickness of the sheet material to approximately one half the thickness of the material in the region of the depression left by the press tool, so that one part of the double flared (concave barrel shaped) opening is removed leaving only one flare (i.e. one half of the barrel shape), so that the remaining opening tapers in one sense only. The resulting shape can be likened to a venturi shape, which can give a beneficial discharge coefficient when the opening is employed to discharge ink as an ink jet nozzle.

By appropriate choice of initial thickness of the sheet material and size of the initial hole and the precise extent of the machining, the eventual size of the hole and the form of the remaining taper can be controlled, in the case of the size to a diameter typically of the order of 18 μm.

According to a further aspect of the invention instead of the workpiece sheet material being placed on a flat unyielding base plate surface, and a single tool being pressed into the material, two tools can be used to apply equal force on opposite faces of the material, thereby pinching the sheet material between the two tools.

One tool may for example be fixed, and the other tool movable, so that after placing the sheet material on the former, the movable tool may be moved into contact with the sheet material to squeeze it between the faces of the two tools.

The size and end face shape of the two tools may be similar or different to produce different effects on squeezing.

Where the two tools present flat end faces of the same area to the sheet material and are axially aligned so that a generally symmetrical stress pattern will be set up in the sheet material around its opening the risk of bowing of the sheet material (which can occur if the area of the tool (or base plate in the case of a single tool) on one face of the sheet material is different from that of the tool or base plate acting on the other face or the tools or tool and base plate are misaligned) can be reduced.

According to a further aspect of the invention the sheet material may be clamped over an area immediately surrounding the oversize hole, between two plates having aligned openings through which upper and lower tools can protrude to engage the upper and lower faces of the sheet material, the latter being positioned so that the initially formed oversize hole is central of the openings, and therefore of the two tools.

Preferably the tools are a clearance or close slipping fit within the aligned openings, so that outward radial spread of the work-piece material is largely prevented, and the size reduction (brought about by the squeezing of the sheet material) is in fact increased since the squeezed material is only free to move radially inwardly to reduce the size of the hole.

Surface deformation of the sheet material beyond the compressed area will also be reduced when employing such an arrangement, since the sheet material around the area impacted by the two tools is gripped between the two plates and if they rigidly clamp the material therebetween there is little tendency for it to deform (as by increasing in thickness) due to any tendency for material to try and move outwardly from the centrally squeezed region.

Where the end product requires a number of holes in an array in close proximity, oversize holes may be pre-formed in the sheet material as by drilling, each centred on where the final smaller holes are required to be positioned in the array, and the sheet material may then be located between a similar array of tools and a base plate (or array of pairs of upper and lower tools) having the same pitch spacing as the pre-formed holes, and the tools forced into contact with the sheet material so as simultaneously to squeeze the latter around each of the holes to reduce their size as aforesaid, thereby to speed up production.

Alternatively a larger area press tool (or a pair of larger area upper and lower press tools) may be employed so as to encompass an area of sheet material containing a plurality of pre-formed holes so that when the sheet material is correctly located relative thereto and squeezed as aforesaid, the diameters of all the holes encompassed by the tool(s) will be reduced simultaneously.

Further variations of the method allow for the production of holes with differing sections and shapes, many of which can be difficult to produce using other processes. For example, squeezing material around circular holes using flat-faced tools will generally produce holes with a waisted cross section, so that each hole reduces in diameter towards the half-way point and increases therebeyond. Using non-flat differently shaped tool faces, and/or otherwise deliberately altering the coefficient of friction at the tool/material interface and/or between clamping plates and the sheet material around the or each hole, can produce different hole cross section shapes.

Non-circular tools can result in the oversize holes becoming non-circular in their reduced size. Thus for example by using a square section tool a circular oversize hole can when reduced in size become a four lobed hole.

According to a further aspect of the invention composite or integrated tooling may be employed comprising a retractable drill centred in a first press tool and a co-operating opposed second press tool between which sheet material can be squeezed as required with or without clamping plates around the press tools, drive means for advancing and retracting the drill relative to the first press tool and for rotating the drill when required, drive means for advancing and retracting the clamping plates independently of the press tools, and drive means for advancing and retracting at least one of the press tools, whereby in use sheet material can be clamped between the clamping plates, the drill advanced and rotated to drill and oversize hole in the sheet material and thereafter retracted fully into the first press tool and thereafter the press tools advanced to squeeze and compress the sheet material and thereby reduce the size of the oversize hole.

The second press tool may be advanced to engage the opposite face of the sheet material from that engaged by the drill bit to provide a support therefor during drilling, in which event the central region of the second press tool may include a depression or cavity to receive the end of the drill as it penetrates the sheet material.

Where the presence of such a depression or cavity could interfere with the subsequent deformation of the sheet material to reduce the size of the hole, the second press tool may include a central cylindrical bore within which is slidable a piston like closure having a flat upper surface which corresponds to that of the end face of the second tool and dive means, which operates for example in synchronism with that for advancing and retracting the drill, operating to retract the piston like closure while the drill is advanced to create a drill receiving cavity but advance to complete the end face of the second tool when the drill is retracted.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows a press tool and base plate for squeezing a sheet material workpiece,

FIGS. 2A to 2C show the sequence of operations for the basic process,

FIG. 3 shows the deformation in sheet thickness which can occur by the process of FIGS. 1 and 2,

FIG. 4 shows the material which can be removed by machining,

FIG. 5 shows how by removing more material the cross sectional shape of the wall of the hole can be effectively modified,

FIGS. 6 and 7 show how two press tools can be employed,

FIG. 8 shows a single position hole-forming machine for performing the method of the invention, and

FIG. 9 shows a two position hole-forming machine for performing the method of the invention.

FIG. 10 shows a four lobed hole formed by a square section tool.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 a press tool body 10 is shown mounted above a fixed base plate 12 on which has been laid a workpiece of stainless steel sheet or foil 14 having a pre-drilled hole 16. The workpiece is positioned so that the hole 16 is central of the workpiece engaging part 18 of the press tool. Tool part 18 is circular in cross section and can be for example hydraulically moved in a downward sense (perhaps against a spring restoring force) relative to the body of the tool 10.

In FIG. 2A the workpiece is shown placed on an unyielding base plate 12, ready to be squeezed as the tool part 18 is moved downwardly to engage workpiece 14. Typically the tool part 18 has a flat underside 19 and is several times larger in diameter than the pre-formed holes 16, typically five times larger in diameter. The tool part 18 is positioned concentrically over the pre-formed hole 16.

FIG. 2B shows the effect on the workpiece 14 as the tool part 18 presses into the sheet material and the latter is displaced radially inwardly and outwardly as shown by the arrows, so as to leave an opening 20 the wall of which can be likened to a concave barrel, albeit of smaller diameter than tool part 18. Friction at the interface between the tool part 18 and the sheet metal 14, and between the sheet metal and the base plate 12, results in the reduced diameter hole 20 assuming a concave barrel shape such that its diameter increases from the midpoint of the material thickness towards each surface of the sheet material.

In FIG. 2C the circular section tool part 18 is shown withdrawn, leaving the deformed workpiece material in which a circular depression 22 around the hole 20 has a reduced thickness and is surrounded by an annular region of increased thickness 24. The material beyond annular region 24 will remain substantially the same thickness as the original workpiece 14.

Subsequent to the tool pressing process the sheet material workpiece has the form shown in cross section in FIG. 3. Displaced material creates the raised ring 24 around the circular reduced thickness region 22 created by the tool part 18. Also shown is the concave barrel shape of the reduced diameter hole 20, which shape is formed by friction present at the tool-workpiece and the base-workpiece interface.

The ring of material 24 can be removed by machining along the line 23, leaving a circular depression 22 around each reduced size hole 20.

FIG. 4 shows hatched at 26 the material which must be removed by machining along the line 25, to leave a uniform thickness component 27 (i.e. without depressions around the holes 20).

FIG. 5 shows how more material (shown hatched at 28) can be removed along the line 31 so as to further reduce the thickness of the remaining sheet material 29, and thereby to remove the upper flare 20 a of the double flared opening 20, so as to leave only the lower flare 20 b. The resulting lower flare 20 b has a generally tapered venturi shape, which can give a beneficial discharge coefficient when used to discharge ink as an ink jet nozzle.

In a variation of the method, instead of the workpiece being located between a flat unyielding base plate 12, and a tool part 18 which is pressed into the workpiece, two similar tools can be used, pinching the workpiece sheet material therebetween. This is shown in FIG. 6 where an upper tool part 18 is opposed by a similar lower tool part 30.

Tool 30 may be fixed and tool 18 may be driven so that it can be brought down into contact with the sheet material workpiece 14, so that it is sandwiched therebetween.

Bowing of the workpiece 14 is reduced due to the symmetry of the forces applied to the sheet material 14 by the two tool parts 18 and 30 as shown in FIG. 6.

In a preferred modification of the FIG. 6 configuration, the workpiece 14 may be clamped between two apertured plates 32, 34 as shown in FIG. 7. The apertures in the plates are aligned as shown at 36, 38 so that upper and lower tool parts 18, 30 can protrude therethrough to engage the upper and lower faces of the sheet material 14. The tools 18, 30 are a clearance or close slipping fit within the openings 36, 38, respectively, so that upward and outward radial spread of the workpiece material is largely prevented. This increases the reduction in size of the hole 16 brought about by the squeezing of the sheet material, since the squeezed material is only free to move radially inwardly to reduce the size of the hole, and also reduces unwanted surface deformation of the sheet material beyond the compressed area.

A drill (not shown) may be located in the upper press tool 18 and a piston like closure may be located in a central bore in the lower tool part 30 so that with the lower tool engaging the underside of the workpiece 14 the drill can be advanced through the end of the upper tool 18 to drill the hole 16 in situ. The end of the drill (which protrudes through the workpiece) can be accommodated in the upper end of the bore in the lower tool 30 if the piston like closure is retracted to leave a cavity centrally of the upper end of the lower tool 30 during the drilling. After the hole is formed, the drill can be retracted, and the closure advanced so that the upper face of 30 is flat and continuous again. The upper tool 18 is then driven down to squeeze the workpiece material 14 to reduce the size of the hole.

Where the workpiece requires a number of holes arranged in an array in close proximity, the sheet material may be pre-drilled as before, and an array of upper and lower tool pairs such as 18, 30 with the same pitch spacing as the pre-drilled holes, may be provided, and after placing the pre-drilled sheet therebetween, the tool pairs may be forced into contact with the sheet material workpiece 14 so as to squeeze the latter around each of the holes simultaneously, to speed up the production process. Alternatively larger area tools may be provided so that a plurality of holes are encompassed between the two opposed tool faces, enabling the said plurality of holes to be simultaneously reduced in size by one operation.

FIG. 8 shows a complete machine for forming a hole in a single position. This comprises a base 40, a lateral arm 46 supported by a rigid upright 44 and a flat anvil 42 on which a sheet workpiece 48 is laid. The centre of the anvil 42 is composed of a single cylindrical piston 50 which is slidingly received in a cylindrical bore 52 in the anvil and is attached at its lower end to a rigid arm 54 which extends laterally from the lower end of a vertical slide 56. The latter is movable in a vertical sense up or down, by an actuator 58—typically a hydraulic or pneumatic ram actuator.

A second rigid arm 60 extends from the side of the vertical slide 56 at its upper end and a drilling spindle 62 is rotatably supported in a bearing 64 and extends vertically below the arm 60 through a hollow punch 66, itself carried in a cylindrical housing 68 which extends vertically below the inboard end of a rigid arm 70 the outboard end of which is acted on by a second actuator 72 for raising and lowering the arm 70 and thereby the punch 66.

The housing 68 is slidably supported in a linear bearing 74 carried on the inboard end of a rigid support arm 76, the outboard end of which extends laterally of and is joined to the upright 44.

The actuator 58 is coupled directly to the machine base 40 and the actuator 72 is coupled to the machine base via the upright 44 so that the vertical slide assemblies 56, 60 and 68, 70 respectively, can be raised and lowered relative to the machine base 40 as required.

Although not shown, a third actuator may be provided to move the punch 66 relative to the housing 68 so that the workpiece is gripped between the anvil 42 and both the punch 66 and the lower face of the housing 68, to reduce the material flow in a radial outward sense during compression.

Rotational drive for spindle 62 is provided by motor 78 and drive belt 80 which engages pulley 82.

In use the spindle is driven in rotation to rotate drill 84 on the lower end of the spindle 62 and the actuator 58 is retracted to lower the drill 84 to engage and drill a hole in workpiece 48. In doing so, arm 54 and piston 50 are similarly lowered relative to the fixed part of anvil 42 so that the drill bit can enter the space left above the piston. When the drilling is completed actuator 58 is extended to lift the slide assembly 56, 60 and raise the spindle and drill clear of the workpiece and since the piston 50 travels up with arm 54, so that its upper face is flush with that of the anvil 42, the upper face of the anvil is now continuous and flat once again.

Thereafter actuator 72 is retracted so that the hollow punch 66 is now forced downwardly to squeeze the workpiece between punch and anvil 42, to reduce the size of the hole.

After the squeeze is complete actuator 72 is extended to lift the punch 66 clear of the workpiece and allow the workpiece 48 to be moved (to form another hole) or to be removed and replaced by another.

An alternative arrangement comprises a drilling station where an oversize hole is drilled in the sheet metal. A pressing station is placed alongside the drilling station, the two stations being at a predetermined spacing from one another. A translating device is provided that is capable of moving the work-piece between the two stations by the predetermined distance. Reference tooling for locating the component is used on the translating table, such that the drilled and reduced holes can be positioned relative to a feature on the component such as for example an edge.

FIG. 9 illustrates this alternative arrangement.

In FIG. 9 the machine includes a base 86 and operator console 88 joined by an umbilical 90. The base carries a workpiece platform 92 which is slidable horizontally relative to the base by a horizontal drive 94. A workpiece 96 is shown mounted on the platform slide 92 and located by a reference edge 93.

The platform slide 92 can move from the position shown, completely to the right-hand end of the base. In its first left-hand position the workpiece 96 is positioned below a drill 98 and in the right-hand position, below a punch tool 100.

The drill is carried at the lower end of a spindle and drive unit 102 which is movable vertically relative to a rigid support structure 104 by a vertical drive 105 which includes a spindle 106 and motor 108.

The punch 100 is carried at the lower end of a carrier 110 which is slidable relative to another rigid support structure 112 and is movable by a similar vertical drive 113 comprising a spindle 114 and motor 116.

Power is delivered to the drive motors as required under the control of an operator standing at the console 88, so that after positioning the workpiece 96 and platform slide 92 correctly below the drill 98 the latter is operated to drill a hole in the workpiece. After raising the drill clear, the workpiece support is moved to the right to position it below the punch 100 and after operating drive motor 116 the workpiece is squeezed between the punch 100 and the platform slide 92 to reduce the size of the hole, whereafter the punch is raised clear, to allow the workpiece to be removed.

FIG. 10 shows the result of using a tool having a square cross section to reduce the size of a round hole. A four lobed hole 120 is formed in a workpiece 122 by striking the workpiece with a tool having a square cross section. The tool produces a square depression 124 in the surface of the workpiece that is equal in size and shape to the square cross section of the tool. The size and location of the predrilled hole prior to being struck by the square cross section tool is shown by the dotted line 126.

Having thus described the invention, various modifications and alterations will occur to those skilled in the art, which modifications and alterations are intended to be within the scope of the invention as defined by the appended claims. 

1. A method of forming a hole of a particular size in sheet material comprising the steps of: forming an initial oversize hole larger than the said particular size in the sheet material, and compressing the sheet material after the hole has been formed, at least in the immediate vicinity of the hole, to cause the sheet material to flow at least in the direction of the hole, whereby the size of the opening is reduced to form an end product.
 2. A method as claimed in claim 1 wherein the oversize hole is formed by drilling.
 3. A method as claimed in claim 2 wherein the oversize hole is on the order of four times larger than the diameter ultimately required of the hole.
 4. A method as claimed in claim 1 wherein the compression is achieved using a press tool and the sheet material is metal foil.
 5. A method as claimed in claim 4 wherein the tool has a flat underside and is several times larger in diameter than the pre-formed hole.
 6. A method as claimed in claim 5 wherein the tool is positioned concentrically over the pre-formed hole before it is pressed into the material.
 7. A method as claimed in claim 5 wherein the tool material is selected so that the tool is not deformed when pressed into the sheet material.
 8. A method as claimed in claim 7 wherein the sheet material is stainless steel and the tool is formed from hardened tool steel or tungsten carbide.
 9. A method as claimed in claim 4 wherein the sheet material is compressed between the tool and a base plate the material for which is selected so that the base plate will not yield under the force applied by the tool.
 10. A method as claimed in claim 9 wherein the tool and base plate are formed from similar material.
 11. A method as claimed in claim 4 wherein the material displacement which occurs during the compression and deformation of the sheet material causes the material generally to flow radially, both outwardly of and inwardly towards, the initial hole with the result that there is a local thinning of the sheet material around the hole, and a reduction in the diameter of the initial hole.
 12. A method as claimed in claim 11 wherein the sheet material which is not compressed is unrestricted and wherein material displaced radially outwardly by the compression produces an annular region of increased thickness around the area subject to compression.
 13. A method as claimed in claim 12 wherein after the compression the sheet material is reduced in thickness by machining so as to remove any annular region of increased thickness so that the whole of the sheet material has the same thickness.
 14. A method as claimed in claim 12 wherein after compression the sheet material is machined to remove material only from the region of locally increased thickness around the hole caused by unrestrained material which has flowed radially outwardly from the area subjected to compression.
 15. A method as claimed in claim 14 wherein after machining a depression is left around the reduced diameter hole corresponding to the shape and size of the press tool.
 16. A method as claimed in claim 15 wherein further machining is performed so as to reduce the thickness of the whole of the sheet material workpiece to the same thickness as that in the depression surrounding the reduced diameter hole.
 17. A method as claimed in claim 16 wherein the machining is performed as a single step to remove the ring of increased thickness and thereafter further material from around any depression created by the compression so that the sheet is of uniform thickness.
 18. A method as claimed in claim 4 wherein friction at the interface between the tool and sheet metal results in the reduced diameter hole assuming a concave barrel shape such that its diameter increases from the midpoint of the material thickness towards each surface of the sheet material.
 19. A method as claimed in claim 18 wherein further material is removed by machining so as to reduce the thickness of the sheet material to approximately one half the thickness of the material in the region of the depression left by the press tool, so that one part of the double flared concave barrel shaped opening is removed leaving only one flare and the remaining opening tapers in one direction only.
 20. A method as claimed in claim 19 wherein the resulting hole corresponds to a venturi shape, which can give a beneficial discharge coefficient when the opening is employed to discharge ink as an ink jet nozzle.
 21. A method as claimed in claim 19 wherein the initial thickness of the sheet material and size of the initial hole and the extent of the machining, are selected so that the eventual size of the hole is of the order of 18 μm in diameter.
 22. A method as claimed in any of claim 4 wherein the compression is obtained using two press tools working in opposition and the two tools are used to apply equal force on opposite faces of the material, thereby pinching the sheet material therebetween.
 23. A method as claimed in claim 22 wherein one tool is fixed, and the other tool is movable, so that in use after placing the sheet material on the fixed tool, the movable tool is moved into contact with the sheet material to squeeze it between the faces of the two tools.
 24. A method as claimed in claim 23 wherein the end face shape of the two tools is similar.
 25. A method as claimed in claim 24 wherein the two tools present flat end faces of the same area to the sheet material and are axially aligned so that bowing of the sheet material is reduced due to the symmetry of the forces applied to opposite faces of the material.
 26. A method as claimed in claim 23 wherein the end face shape of the two tools is different to produce different effects on squeezing.
 27. A method as claimed in claim 1 further comprising the steps of clamping the sheet material over an area surrounding the oversize hole between two plates having aligned openings through which upper and lower press tools can protrude to engage the upper and lower faces of the sheet material, positioning the lower press tool so that the initially formed oversize hole is central of the openings, and therefore of the two press tools, whereby the outward radial spread of the sheet material is largely prevented.
 28. A method as claimed in claim 27 wherein each of the tools is dimensioned to be a clearance or close slip fit within the openings, and the hole size reduction is increased since the squeezed material is only free to move radially inwardly thereby to reduce the size of the hole.
 29. A method as claimed in claim 28 wherein the sheet material is rigidly clamped between the two plates, and surface deformation of the sheet material beyond the compressed area is reduced, since the sheet material around the area impacted by the two tools is prevented from being deformed as by increasing in thickness due to the outward flow of material from the centrally squeezed region.
 30. A method of forming an array of holes in close proximity to one another in sheet material comprising the steps of pre-forming oversize holes in the sheet material, centering each pre-formed hole in a position where a final smaller hole is required, locating the sheet material between an array of tools and a base plate having the same pitch spacing as the pre-formed holes, and forcing the tools into contact with the sheet material so as to simultaneously squeeze the sheet material around each of the holes to reduce their size.
 31. A method as claimed in claim 30 wherein each initial hole is circular in cross section and a square cross section tool is employed to reduce the size of each hole so that the cross sectional shape of each final hole becomes a four lobed hole.
 32. Apparatus for performing the method of claim 1 comprising means for compressing sheet material between two opposed substantially flat faces of two members at least one of which is movable towards and away from the other, drive means for effecting the said movement, and control means for controlling the operation of the drive means.
 33. Apparatus as claimed in claim 32 wherein both members are movable by drive means.
 34. Apparatus as claimed in claim 32 wherein at least one of the members is a tool which is surrounded by a separate clamping member which is movable relative to the other clamping member to grip sheet material therebetween around the area which is to be compressed by the tool, and drive means for effecting the clamping is provided and the two drive means are controlled as required to clamp and compress the sheet material.
 35. Apparatus for performing the method of claim 1 comprising a retractable drill centered in a first press tool and a co-operating opposed second press tool between which sheet material can be squeezed as required to reduce the size of a drilled hole, drive means for advancing and retracting the clamping plates independently of the press tools, further drive means for advancing and retracting the drill relative to the first press tool and for rotating the drill when required, separate drive means for advancing and retracting at least one of the press tools, and control means for controlling the drives and the operation of the drill, whereby in use sheet material can be clamped between the clamping plates, the drill can be advanced and rotated to drill an oversize hole in the sheet material and thereafter retracted fully into the first press tool, and thereafter the press tools can be advanced to squeeze and compress the sheet material and thereby reduce the size of the oversize hole, whereafter the drive means are operated to allow the sheet material to be moved.
 36. Apparatus as claimed in claim 35 wherein in use the second press tool is advanced to engage the opposite face of the sheet material from that engaged by the drill bit to provide a support therefor during drilling and the central region of the second press tool includes a depression or cavity to receive the end of the drill as it penetrates the sheet material.
 37. Apparatus as claimed in claim 36 wherein the second press tool includes a central cylindrical bore within which is slidable a piston like closure having a flat upper surface which corresponds to that of the end face of the second tool, and drive means which operates in use to retract the piston like closure while the drill is advanced to create a drill receiving cavity and to advance the closure so as to be flush with the end face of the second tool when the drill is retracted.
 38. Apparatus for performing the method of claim 1 comprising a drilling station, where the oversize hole is drilled in a sheet material component, a pressing station alongside the drilling station, the two stations being at a predetermined spacing, a translating device capable of moving the component work-piece between the two stations by the predetermined distance, and reference tooling for locating the component on the translating table, such that the drilled and reduced hole will be positioned relative to a feature on the component such as an edge thereof. 